France warned Britain at the start of a European Union summit on Thursday that it would face a tough, unyielding opponent if it sought too many concessions during its negotiations to leave the 28-nation EU. British Prime Minister Theresa May, who was attending her first EU summit as leader, was expected to brief her European counterparts on the way ahead for Britain’s exit from the trade bloc.

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Many uncertainties about the divorce remain because Britain has yet to trigger the two-year negotiations for “Brexit” and is unlikely to do so until the end of March.

“It’s in the interests of the UK and the EU that we continue to work closely together, including at this summit,” May said.

French President Francois Hollande immediately insisted that the EU would not surrender the bloc’s core values just to keep Britain close as a future ally.

“I have said so very firmly: Mrs. Theresa May wants a hard Brexit? The negotiations will be hard,” Holland said.

Britain’s June 23 breakup referendum has forced the 27 other nations to plot their future without a major but often recalcitrant member state.

European leaders have grumbled that Britain’s tardiness in starting the negotiations slow down their own planning for the next few years.

German Chancellor Angela Merkel said the other leaders would underscore the urgency and would “make clear again: We are waiting for the notification from Britain.”

The Brexit referendum to leave the EU was a milestone in the history of the bloc and the disentanglement is expected to be long, difficult and confrontational.

Still, EU President Donald Tusk, who chairs the summit, assured the British leader she would not get too hostile a reception.

Tusk said that some compared it to “entering the lion’s den. It’s not true. It’s more like a nest of doves. She’ll be absolutely safe with us. And I hope that she will also realize that the European Union is simply the best company in the world.”

May said Britain would continue to be a responsible member right up to the day it leaves the EU. “I’m here with a very clear message. The UK is leaving the EU but we will continue to play a full role until we leave.”

Like Hollande, Tusk vowed last week not to compromise on the bloc’s principles in negotiating Britain’s departure and warned that London is heading for a hard exit.

He insisted that Britain can’t hope to both stay in Europe’s single market and restrict the movement of EU migrants, saying there would be no compromises.

May has appeared to signal that her government would prioritize controls on immigration over access to the European single market, an approach informally called a “hard Brexit.”